About dust extraction systems
Main functions of dust extraction system:
better working conditions and protection of workers,
protection of the environment and machines,
recycling of waste material.
Elements of each dust system are an air suction system, a filter, a silo, or other tank, and automatics with control sensors and safety sensors.
The dust suction system is made up of fans and piping which it flows polluted air which is passing through the filter. Depending on the process, the filtered dust is transported to the silo by transport pipelines and the purified air can be returned to the production hall again. Picture 1. shows the main parts of the dust extraction system and picture 2. shows installed dust extraction system.
Picture 1: Elements of a single system
Picture 2. Installed dust extraction system – PELET GRUPA, Novska
Depending on the type and amount of suction and filtered dust, there are types of filtering devices. The main division is positive pressure and negative pressure systems.
The choice of a filter device depends solely on the load on the system and the type of dust (abrasive, non-abrasive, explosive, etc.). The loading of the system depends on the amount of dust and the content of the smallest particles in the extracted material. The greater the total dust content and / or the proportion of finer particles, the greater the load.
In the case of non-abrasive dust, a pressure system (pressure filter) is used. The material passes through the fan and in the filter the bulk of the dust is separated gravitationally and the smaller part on the filter surface. Due to the low load, such filters, if well selected, work without additional shaking. If the specific filter loads are large, which is typical of abrasive dust in the wood industry, and almost always in cement, food, chemical and other industries, pressure filters are supplemented with bag shaking (regeneration) systems. There are three ways to shake a filter bag, which are:
- vibration shaking
- counterpressure shaking fan
- shaking bags with compressed air,
Shaking mode is also selected depending on the system load and dust properties. Vibrating shaking is the simplest but also the least effective. The most effective shaking of the bags is by compressed air, which is programmed, in large quantities, into a clean interior of the bag in a very short time and, when passing through it, strongly escapes dust particles accumulated on the outer surface of the bag. The same principle applies to blower fan shaking, only to be less effective. If the dust that is filtered is very abrasive, the pressure system must be applied to ensure durability of the equipment. In these systems, the entire transport of material from the point of origin to the filter is under pressure, and the fan unit is located behind the filter and passes through the already filtered, dust-free air. This protects the fan and prevents the pipeline from coming in contact with the abrasive dust except the knees.
Negative pressure systems are often applied even where concentrations of explosive gas or dust mixtures are possible. Characteristic of underpressure systems is that the filter must have a design resistant to the created underpressure and, as a rule, have one of the bag shake systems installed.
Picture 3. Example of negative pressure system –DALSTROJ ADRIA WINCH, Split
Picture 4. Example of positive pressure system –VEKTRA JAKIĆ, Pljevlja
Separated dust in the filter should be removed from the filter as soon as possible to prevent it from clogging or accumulating a large amount of entrained dust, which in some conditions presents a high risk of fire and explosion.
For lightly loaded filters, as well as for filters in industries where the footprint is very dusty, it is sometimes sufficient to use PE bags or a pressurized bin (metal container) as a container. For larger units, the discharge of the material is solved by a chain or auger transport via a rotary air inlet (dispenser) which seals the filter with pressure, and gravity discharges the material gravitationally from the filter.
Depending on the further use of the filtered material and the automation of the plant, the material is transported to a silo or container or directly fed to the boiler room or briquette.
For all of these filters to work, there are electric motor drives, then a variety of sensors and data encoders to match the priority of operation, control of safety and very high requirements for the implementation of fire and explosion protection. If the frequent present installations of compressed air for electromagnetically controlled valves are added here, then there is a need for automation with a more complex filter to accomplish all these functions.